Filters are designed, for example, for a specific frequency band. Signals within that specific frequency band will pass the filter from a first port to a second port whereas signals outside of the specific frequency band will be prevented from passing through the filter. The filters may be produced as a combination of resistors, inductors, and capacitors or as ceramic filters, such as but not limited to surface acoustic wave filters or film bulk acoustic resonators. Dependent on their technology each filter may be distinguishable from other filters by electrical characteristics such as insertion loss, out-band signal rejection, electric power durability, but also in size and price.
A special application of filters is a duplex filter for radio transceivers. A duplex filter enables a transmitter and a receiver to share a single antenna, by preventing a transmission signal from entering the receiver and, likewise, the received signal from entering the transmitter. The duplex filter usually comprises two separate band-pass filters, one of which is connected to the receiver section of the transceiver, and the other filter being connected to the transmitter section of the transceiver. Duplex filter designs are commercially available as off-the-shelf filters for a plurality of different circuit designs for the transceiver. However, the technical properties of these off-the-shelf filters may not always meet the design criteria a circuit designer has in mind when designing a radio station.
The use of mobile communications networks has increased over the last decade. Operators of the mobile communications networks have increased the number of base stations in order to meet an increased demand for service by users of the mobile communications networks. The operators of the mobile communications network wish to purchase components for the base stations at a lower price. As the size and price of radio stations goes down, there is a need to provide, smaller and less expensive filters and duplex filters.